The present invention relates to a ground station for a VOR navigation system with at least one transmitter and several radiators which ground station radiates azimuth-dependent and azimuth-independent information, the azimuth-dependent information consisting of the wave amplitude-modulated with 30 Hz in the field and generated by the superposition of the nondirectionally radiated VHF carrier on a VHF figure-8 directional pattern rotating at 30 Hz, and the azimuth-dependent reference wave consisting of a 30-Hz wave which is contained as frequency modulation in a 9,960-Hz subcarrier wave amplitude-modulated on the VHF carrier.
Such ground stations as known in the prior art are described in a book by E. Kramar, "Funksysteme fur Ortung und Navigation", Verlag Berliner Union GmbH, Stuttgart (1973), on pages 131 to 137.
The VOR navigation system operates in the frequency region from 108 to 118 MHz. The channel spacing is 100 kHz. As a result of the increase in air traffic, more and more VOR stations are being installed; as a consequence, the number of channels is no longer sufficient. To be able to install more VOR ground stations in the available frequency region from 108 to 118 MHz, the channel spacing must be reduced to 50 kHz.
With this reduced channel spacing, disturbances may occur which are not present with the present channel spacing of 100 kHz. The possible disturbances occur under the following conditions:
A. An airborne receiver has been set by mistake to an vacant adjacent channel separated by 50 kHz. In this vacant adjacent channel, the harmonics of the modulation frequency which fall within this channel may still have such amplitudes as to result in a stable, but false indication. There may be no indication of the disturbance by the flag alarm.
B. Navigation is to be effected with the aid of a very distant VOR station. The weak signal of this very distant VOR station and the above-mentioned harmonics of a close VOR station are in the same channel. There may be a superposition of the two signals and, consequently, a false indication.
These disturbances (anomalies) will be explained in more detail in the description.